1. Field of the Invention
This invention relates to a control system for a four-wheel-steered vehicle provided with a slip control means such as an antilock brake system or a traction control system.
2. Description of the Prior Art
Recently there has been put into practice a four-wheel-steered vehicle in which both the front and rear wheels are turned when the steering wheel is turned. In such a four-wheel-steered vehicle, the rear wheels are turned according to predetermined rear-wheel turning characteristics which determines the rear wheel wheel turning angle ratio (the ratio of the rear wheel turning angle to the front wheel turning angle). Generally, the rear wheels are turned in the same direction as that of the front wheels when the vehicle speed is relatively high and in a direction opposite to that of the front wheels when the vehicle speed is relatively low. In this specification, that the rear wheels are turned in the same direction as that of the front wheels will be expressed as "the rear wheels are turned in the same phase", and that the rear wheels are turned in a direction opposite to that of the front wheels is expressed as "the rear wheels are turned in the reverse phase". When the rear wheel turning angle ratio is positive, the rear wheels are turned in the same phase and when the rear wheel turning angle ratio is negative, the rear wheels are turned in the reverse phase.
Further, there has been known an antilock brake system (to be abbrevitaed as "ABS", hereinbelow) in which braking force is controlled to prevent lock of the wheels when the brakes are applied.
The ABS detects the rotational speeds of all the wheels and calculates the vehicle speed and the slip ratio of each wheel on the basis of the rotational speeds of the wheels. Then the ABS controls the brake fluid pressure so that the slip ratio is kept at a value which will provide the most efficient braking, thereby preventing lock of the wheel. More particularly, the ABS calculates a target vehicle speed to which the vehicle speed is expected to decrease when the vehicle is braked at a target slip ratio which will provide the most efficient braking, and when the slip ratio reaches a predetermined braking-control starting threshold value, the ABS controls the braking force or the brake fluid pressure so that the peripheral speed of the wheel conforms to the target vehicle speed. (See Japanese Unexamined Patent Publication No. 56(1981)-135358, for instance.)
It is said that the slip ratio which will provide the most efficient braking (This slip ratio will be referred to as "the most effective slip ratio", hereinbelow.) is about 15 to 20%. However when said target vehicle speed is determined on the basis of such a slip ratio, the wheels can easily loose gripping force when the brakes are applied during cornering where a cornering force is required, though the most efficient braking may be obtained when the brakes are applied while the vehicle is running straight. That is, during cornering, the steering wheel is turned by an angle which is larger than the angle required to direct the vehicle in a desired direction so that a larger angle of skid is provided to the wheels and the component of the gripping force produced by the angle of skid in the rotating direction of the wheels which is directed toward the center of the turn acts on the wheels as the cornering force. As a result, the gripping force in the rotating direction of the wheels is reduced by the cornering force, and accordingly, when the braking force is controlled during cornering with the most effective slip ratio employed as the target slip ratio as when the vehicle is running straight, the wheels are easily locked since the component of the gripping force in the braking direction is smaller than when the vehicle is running straight.
Accordingly, the target slip ratio is generally set below the most effective slip ratio taking into account braking during cornering.
In the case of the four-wheel-steered vehicle described above, a larger cornering force is obtained when the rear wheels are turned in the same phase. Accordingly, when the rear wheels are turned in the same phase, the target slip ratio can be enlarged so that braking efficiency is improved. On the other hand, if the rear wheel turning angle ratio is corrected on the basis of the slip ratio of the wheels detected by the ABS so that a required cornering force is obtained, the target slip ratio for the braking force control may be large or equal to the most effective slip ratio from the first.
However when the rear wheel turning angle ratio is corrected and the target slip ratio is changed after the ABS begins controlling the braking force during cornering, the braking effect fluctuates during cornering, which is sometimes dangerous.
Accordingly, it is preferred that the rear wheel turning angle ratio be changed before the ABS begins controlling the braking force.
Further, there has been known a traction control system which controls the driving force of the driving wheels when the slip ratio of the driving wheels with respect to the road surface exceeds a preset slip ratio. (See Japanese Unexamined Utility Model Publication No. 60(1985)-60356, for instance.)
Since when the driving force is controlled, part of the driving force is lost, it is preferred that the preset slip ratio be as large as possible. However, when the slip ratio of the driving wheels increases, cornering properties of the vehicle are apt to change especially on a slippery road surface. For example, in a front-wheel drive car, understeer tendency is enhanced when the slip ratio of the front wheels increases, and a in a rearwheel drive car, oversteer tendency is enhanced when the slip ratio of the rear wheels increases. Accordingly, conventionally the preset slip ratio has been generally set somewhat small taking into account the change in the cornering properties.
In the case of the four-wheel-steered vehicle, by properly correcting the rear wheel turning angle ratio to compensate for the change in the cornering properties, the preset slip ratio can be relatively large and the loss of the driving force can be reduced.
However when the correction of the rear wheel turning angle ratio is effected simultaneously with the initiation of the traction control, the driving force must be abruptly controlled immediately after the initiation of the traction control since the slip ratio of the driving wheels has had a tendency to quickly increase by the time the traction control is initiated, which leads to other loss of the driving force.
Accordingly, it is preferred that the rear wheel turning angle ratio be changed before the traction control system begins controlling the driving force.
As can be understood from the description above, in the four-wheel-steered vehicle provided with a slip control means such as an ABS or a traction control system, it is preferred that the rear wheel turning angle ratio be changed prior to initiation of slip control by the slip control means.